1. Field of the Invention
The present invention generally relates to an internal combustion engine control apparatus for controlling the timing such as ignition timing and/or fuel injection timing for each of individual engine cylinders on the basis of a reference position signal indicating first and second reference positions. More particularly, the invention is concerned with an engine control apparatus which can ensure the timing control with high accuracy even when the engine is in the transient operation state.
2. Description of the Related Art
In general, in the internal combustion engines (hereinafter referred to simply on the engine) such as those for automobiles or motor vehicles, there is employed a microcomputer-based control means for calculating the ignition timing and the fuel injection timing for each of the engine cylinders on the basis of reference angular positions of a crank shaft of the engine in order to control optimally the fuel injection timing and the ignition timing while taking into account the operation state of the engine.
The reference positions for such timing control are derived from the reference position signal. In this conjunction, it is noted that in actuality, the timing control is effectuated with a delay corresponding to one interval between the reference position signals from a time point at which the preceding reference position signal was detected. Accordingly, it is desirable to perform the timing control by predicting the succeeding reference position. This is true in particular when the engine is in the transient state of operation such as acceleration and deceleration. In that case, it is desirable or necessary to detect the trend of change in the period of the reference position signal and determine the predicted reference position period for thereby correcting the control timing.
For a better understanding of the present invention, the background technique thereof will first be elucidated.
FIG. 7 is a block diagram showing schematically a structure of an engine control apparatus known heretofore. The control apparatus is comprised of a reference position signal generating means 1 for generating a reference position signal T.theta. which indicates first and second reference positions (described later on) on a cylinder-by-cylinder basis in synchronism with the engine speed (rpm), a variety of sensors for detecting operation states D of the engine and denoted generally by a numeral 2, and a control means 3 realized by a microcomputer for controlling the engine operation on the basis of the reference position signal T.theta. and the engine operation state signal D.
The control means 3 includes a reference position period measuring part 31 for measuring a reference position period T180 on the basis of the reference position signal T.theta., a predicted reference position period calculating part 32 for calculating a succeeding period, i.e., a predicted reference position period Tex on the basis of change or variation in the reference position period T180, and a timing controller 33 for controlling the engine on the basis of the predicted reference position period Tex, the reference position signal T.theta. and the engine operation state signal D.
The timing controller 33 recognizes the reference position for each engine cylinder on the basis of the reference position signal T.theta. and calculates the control timing (ignition timing, etc) while taking into account the engine operation state D, corrects the control timing on the basis of the predicted reference position period Tex, and outputs a control signal corresponding to the corrected control timing. The control signal outputted from the timing controller 33 is supplied to means for controlling an ignition coil and a fuel injector (not shown).
FIG. 8 is a perspective view showing in concrete an exemplary structure of the reference position signal generating means 1. As can be seen from this figure, this reference position signal generating means 1 is constituted by a signal disk 11 mounted on a cam shaft 10 which is rotated in synchronism with the engine operation. The signal disk 11 includes a number of slits 12 formed coaxially and extending in the direction in which the signal disk 11 is rotated, wherein the number of the slits corresponds to that of the engine cylinders. Each of the slits 12 has a leading edge corresponding to the first reference position and a trailing edge corresponding to the second reference position. Of these slits 12, the one corresponding to a particular one of the cylinders has an offset at the leading edge. A light emission element 13 such as a photodiode and a light receiving element 14 such as a phototransistor are disposed in opposition to each other so that the slits 12 for generating the reference position signal T.theta. pass between the elements 13 and 14, which thus cooperate to constitute a photodetector which generates a pulse of the reference position signal T.theta. every time the slit 12 passes by.
FIG. 9 is a timing chart showing a waveform of the reference position signal T.theta., wherein T180(n-n) represents the preceding reference position period (i.e., period intervening between the first reference positions), while T180(n) represents the current reference position period with a symbol Tex representing the succeeding reference position period as predicted. Each of these reference position periods corresponds to 180.degree. in terms of the crank angle.
The reference position signal T.theta. includes a pulse which rises up at the first reference position B75.degree. (meaning the crank angle 75.degree. before the top dead center or TDC) and falls at the second reference position B5.degree.. The first reference position may also be referred to as the maximum angle-of-advance position, while the second reference position may be termed as the initial or angle-of-lag reference position. Each pulse has a duration of "H" level corresponding to 70.degree. in terms of the crank angle.
Next, operation of the conventional engine control apparatus shown in FIG. 7 will be described by reference to FIGS. 8 and 9.
Operation of the engine causes the cam shaft 10 to rotate, as the result of which the reference position signal generating means 1 generates the reference position signal T.theta. of such a waveform as shown in FIG. 9. The various sensors denoted collectively by the numeral 2 generate various engine operation state signals such as engine speed (rpm), load and so forth. These signals are inputted to the control means 3 together with the signal T.theta..
In the steady state of operation of the engine in which the engine speed remains substantially constant, the timing controller 33 incorporated in the control means 3 calculates the ignition timing and the fuel injection timing on the basis of the reference position signal T.theta. and the operation state signal D by using the predicted reference position period Tex as the reference position period T180(n) because the predicted reference position period Tex is equal to the current reference position period T180(n) in the steady operation state of the engine.
By way of example, let's consider the ignition timing control. In a high-speed operation of the engine where the ignition timing has to be controlled with an advanced angle, the timer control of the ignition timing is performed by reference to the first reference position, while in a low-speed operation where the ignition timing is controlled with a lag, the timer control of the ignition timing is carried out by reference to the second reference position B5.degree.. At this juncture, it should be noted that electric power distribution to the individual engine cylinders is realized mechanically through discharge electrodes (not shown) mounted on a rotating shaft.
On the other hand, the reference position period measuring part 31 incorporated in the control means 3 measures the period of the reference position signal T.theta. from the first reference position B75.degree. as the reference position period T180. On the other hand, the predicted reference position period calculating part 32 calculates the succeeding reference position period Tex as the predicted reference position period on the basis of the preceding and current reference position periods T180(n-1) and T180(n) in accordance with EQU Tex=T180(n)+K{T180(n)-T180(n-1)} (1)
In the above expression (1), the term "T180(n)-T180(n-1)" represents a deviation .DELTA.T from the preceding reference position period, and K (.apprxeq.1) represents a prediction weighting coefficient. The prediction weighting coefficient K is set to an optimal value determined by taking into account the acceleration characteristics and other factors intrinsic to the engine of concern.
As can be seen from the above expression (1), such arrangement may be adopted in which the deviation .DELTA.T is multiplied by the prediction weighting coefficient, and the product resulting from the multiplication is added to the current period T180(n) for thereby determining the predicted reference position period Tex.
Next, assuming that the engine is in the transient state such as, for example, acceleration state, the current reference position period T180(n) becomes shorter than the preceding period T180(n-1), wherein the difference therebetween is reflected onto the next predicted reference position period Tex.
At this time, an ignition timing control time Ta can be given by EQU Ta=(.theta.a/180)Tex (2)
where .theta.a represents a crank angle corresponding to the control time intervening between the reference position for the control and the ignition time point Ta (see FIG. 2).
In this manner, the timing controller 33 calculates the control timing on the basis of the reference position signal T.theta. and the operation state D, to thereby output an ultimate control signal corrected on the basis of the predicted reference position period Tex. This control signal represents the control time Ta corrected with the predicted reference position period Tex. Thus, an optimal ignition timing control can be realized.
However, with only the measurement of the reference position period T180, it is impossible to detect variation in the reference position period T180 itself. This in turn means that the engine can not be derived with high accuracy and high efficiency solely by resorting to the measurement of the reference position period T180 only, the reason for which may be explained by the fact that when a rapid or abrupt change occurs in the ratio between a first interval period extending from the first reference position B75.degree. to the second reference position of B5.degree. and a second interval period extending from the second reference position B5.degree. to the first reference position B75.degree., such rapid change can not be reflected onto the succeeding timing control. Besides, it is difficult to realize a matching of the weighting coefficient K to such rapid change by prediction, which may result in that the timing control signal suffers from error because of the reflection of the predicted reference position period Tex.
As will now be appreciated from the foregoing, the engine control apparatus known heretofore suffers from a problem that the apparatus can not cope with a rapid change in the reference position period T180 because the change in the period of the reference position signal T.theta. is corrected by detecting the change in the reference position period T180, whereby the timing control signal is generated on the basis of the reference position signal which suffers from error due to the change or variation mentioned above. Thus, with the known engine control apparatus, it is impossible to realize the control with high accuracy.